Even if we assume your assumption is correct (Lupul-Bozak-Kessel being the worst top line in the league defensively) and we assume that "most casual hockey fans" are correct that Kessel is the worst on the line, that does not necessarily mean he is the worst forward in the league defensively.

There could easily be a worse player defensively on another top line that has defensive stalwarts covering for him that Kessel does not have and that's not even considering lines 2 through 4 throughout the league.

I'll agree that Kessel might not be the worst defensive forward in the league, but your defense of him is: "whoa, hey there! There could be worse guys. We just don't know that because maybe their linemates aren't as crappy defensively as Kessels are?" I'm sure the Leafs thank you for charging to the rescue.